Pressure-electric transducers with performance stabilization



March 12, 1968 J- R. BAILEY 3,373,343

PRESSURE-ELECTRIC TRANSDUCERS WITH PERFORMANCE STABILIZATION Filed Nov. 24, 1965 2 Sheets-Sheet 1 1 INVENTOR JAMES R. BAILEY ATTORNEYS March 12, 1968 J. R, BMLEY 3,373,343

PRESSURE-ELECTRIC TRANSDUCERS WITH PERFORMANCE STABILIZATION Filed Nov. 24. 1965 2 Sheets-Sheet 2 6/ OUTPUT 4 4vuc SUPPLY 46 5/} PRESSURE SENSITIVE RESISTOR INVENTOR. B$AMES R. BAILEY $55756? 45 Bis/$70k M yzAl AT TORNEYS United States Patent 3,373,343 PRESSURE-ELECTRIC TRANSDUCERS WITH PERFORMANCE STABELIZATION James R. Bailey, Milwaukee, Wis, assignor to Johnson Service Company, Milwaukee, Wis, a corporation of Wisconsin Filed Nov. 24, 1965, Ser. No. 509,582 14 Claims. (Cl. 323-49) This invention relates to improvements in pressureelectric transducers with performance stabilization.

Pressure-electric transducers of the type shown and described in Hilgert application, Ser. No. 389,435, now patent No. 3,278,873, filed Aug. 13, 1964, are designed to convert any 0 to 20 psi. input pressure change into a linear voltage or current output change over a range of 0 to milliamperes or 0 to 2 volts. This output is independent of the input supply voltage level. In this type of transducer an increased input pressure will cause a transfer lever to compress carbon or other pressure variable contacts and thus reduce their resistance. The resultant increase in current is then rfed through a torque motor in such a way that the increased feedback torque balances the input force. The current in the circuit is therefore proportional to the input pressure. While this type of device works well for many purposes, there are certain applications where an improvement in stability would be very advantageous. In the transducer of the aforementioned pending application the carbon contacts have a hyperbolic resistance/ pressure characteristic curve -that is, the voltage gain across the contacts varies greatly from low to high pressure, tending to cause instability in the transducer at the low pressure-high gain portion of the curve, and sluggishness at the opposite end of the curve. Also, there is a great resistance change at the time when the drop across the carbon contacts is the greatest, thus magnifying the instability of the instrument at low pressures. Some improvement has been obtained by adding a resistance in series with the carbon contacts. When this is done the gain at low pressures is reduced inasmuch as the resistance change of the contacts is a smaller portion of the total circuit resistance. This, how ever, does not answer the problem, as the voltage change across the contacts is just as great, rising to the full supply voltage at zero pressure and dropping to near zero at high pressure. Another problem with prior instruments of the type discussed is that when a DC current to an inductive load is interrupted, the subsequent collapse of the magnetic flux causes a higher counter EMF to be developed across the open contacts. This counter EMF can cause rapid wearing of the contacts. Utilizing the contacts in a so-called dry switching circuit can cause other difiicultics such as the building up of a non-conducting film or the accumulation of dust on the contact surfaces.

It is a general object of the present invention to improve the stability of pressure-electric transducers of the type described by providing novel means for maintaining a constant low voltage drop across the contacts, this limiting of the voltage drop also serving to minimize the wear on the contacts.

A further object of the invention is to provide a device as above described wherein there is transistor mechanism, arranged in a novel manner, in conjunction with the contacts and feedback coils to provide for a constant low voltage drop across the contacts.

A further object of the invention is to provide as one form of the invention, for uses where only low current output is required, a novel arrangement wherein the contacts, feedback coils, and transistor are all in series so that the contacts carry full load current. Thus no dust or film build-up can occur and any dust is quickly dissipated at each contact operation.

A further object of the invention is to provide as another form of the invention a novel arrangement of transistor mechanism, feedback coils and contacts wherein the output current does not pass through the contacts, thus permitting a transducer to be manufactured which has a higher current output than the contacts could endure.

With the above and other objects in view, the invention consists of the improved pressure-electric transducer with performance stabilization, and all of its parts and combinations, as set forth in the claims, and all equivalents thereof.

In the accompanying drawings, illustrating several embodiments of the invention, in which the same reference numerals designate the same parts in all of the views:

FIG. 1 is a top plan view of a pressure-electric transducer;

FIG. 2 is a sectional view taken approximately on the line 2-2 of FIG. 1;

FIG. 3 is a sectional view taken approximately on the line 33 of FIG. 2;

FIG. 4 is a diagrammatic view showing the wiring for the different sets of coils;

FIG. 5 is a wiring diagram showing one improved circuit, specially adapted for low current output, and including a novel arrangement of devices for stabilizing performance; and

FIG. 6 is a wiring diagram showing another circuit adapted for higher current output and utilizing two transistors.

The present invention is useful in stabilizing the performance of any pressure-electric transducer having pressure responsive contacts of the type where the resistance to flow of current decreases proportionately to the amount of pressure, such as the pressure-electric transducer of Patent No. 3,155,896 dated Nov. 3, 1964. However, for purposes of illustrating a preferred use of the present invention the transducer of co-pending Hilgert application, Ser. No. 228,997, now abandoned, has been illustrated in FIGS. 1-4, inclusive. In this transducer there are spaced parallel elongated base members 11 and 12 formed of suitable metal. One pair of cores comprises the cores 13 and 14, and the core 13 has its lower end swaged into the base 11, and the core 14 its lower end swaged into the other base 12. Similarly, another pair of core members comprises the cores 15 and 16, the core 15 being swaged into the base member 11 and the core 16 into the base member 12. Thus, each base member connects opposite cores of the pairs, and it is to be noted that there is no non-magnetic gap between the lower ends of the cores and the base members.

The cores are each wound with coils as indicated at 17, 18, 19 and 20 in FIG. 4. The design makes it practical to have relatively small cores so that a maxlmum number of turns for the allowable coil resistances is possible.

The pair of cores 13-14 is adapted to coact with an armature 21, and the pair of cores 1546 with an armature 22. The hysteresis is kept to a minimum by the use of low hysteresis core material and base material, preferably an nickel alloy, such as Carpenter HYMU 80 or Allegheny Mumetal. The design is such that it is a relatively simple matter to keep all of the upper pole faces fiat and in a single plane and, inasmuch as these faces are all in a single plane, the structure is relatively simple to produce by means of a common grinding operation. The accurate location of the pole faces in a single plane allows the working air gaps 23 for one pair and 24 for the other pair to be set at small values. This results in the maximum obtainable sensitivity from this factor. The shorter the gaps 23-24 the better, but for practical purposes, it is difficult to have a working gap of less than .002 inch. If the working gap exceeds .010 inch, the main advantages of the present invention are dissipated. For practical purposes the air gaps are maintained between .004-.008 inch. Pole face enlargements may readily be attached to the upper ends of the cores as desired without requiring any significant change in the magnetic structure.

In this transducer of the co-pending application a polarizing or biasing permanent magnet 26 connects the base 11 with the base 12 and is located between the pairs of cores and parallel to the armatures 21 and 22, and the design makes it practical to utilize a single polarizing magnet to serve both pairs of cores.

The armatures may be movably supported as is illustrated in FIGS. 1 and 2, wherein there is a lever 27, preferably of aluminum and preferably U-shaped in crosssection, which is supported to pivot on an axis which extends transversely of the base members 11 and 12 and which is located between the pairs of cores 1314 and 1516. Whenever linearity between the input and output of the device is required, the location of the pivot is midway between the pairs of cores as illustrated. With this arrangement the armatures 21 and 22 are suitably connected to the lower faces of the lever 27 as illustrated. In this design it is to be noted that the cores and, therefore, the length and diameter of the coils 17, 18, 19 and are independent of armature dimensions.

This transducer includes a suitable supporting base 29 on which the magnet bases 11 and 12 are supported. In addition, the base supports spaced posts 30. To the upper ends of the posts is a fiexure hinge 31 which is L-shaped in cross-section and which has a vertical flange secured to the posts and a horizontal flange secured to the underside of the lever 27. This provides the hinge for mounting the lever to pivot on the axis 28.

The base also supports a pressure actuator 32 having an internal pressure chamber 33 for receiving a pressure fluid from an inlet conduit 34. Variations in pressure, constituting the signal from an external source such as a pneumatic thermostat, is admitted to the chamber 33 through the inlet 34 to act on a diaphragm 35. The latter in turn acts on the lower end of a spring 36, and the upper end of the spring is seated against the head of an adjustment screw 37 which is threadedly supported in the transfer lever 27. Another adjustment screw 38 at the end of the transfer lever coacts with the end of a screw 39 to form an adjustable stop.

At the opposite end of the transfer lever an upper contact member 40 is adjustably carried. Bonded to the lower end of the contact member is a carbon disc 41. Suitably supported in and projecting upwardly from the base 29 is a lower contact member 42 having a carbon disc 43 bonded to its upper end. A flexible damping sleeve 44 surrounds the meeting ends of the upper and lower contacts and the carbon discs. With this arrangement, when no pressure is being applied to the contacts the carbon discs 41 and 43 are barely in contact, or very slightly separated, so that there is a high resistance to the flow of electricity. When pressure is applied, however, to the carbon discs, the resistance to the flow of current decreases proportionately to the amount of pressure. Flow of current from the source through the coils 17, 18, 19 and 20 is under the control of these contacts, as will be hereinafter explained more fully,

By referring to FIG. 4 it is to be noted that there is a split flux from the permanent magnet 26, with one flux path passing from the north pole of the permanent magnet, through a portion of the base member 11, through the core 13, through the armature by way of the air gaps, down through the core 14, through a portion of the base 12, and back to the south pole of the permanent magnet 26, as is illustrated in the lower portion of FIG. 4. Another flux path travels similarly from the permanent magnet through the other pair of cores 1546 and the 4 armature 22, as is illustrated in the upper portion of FIG. 4. It is to be noted that the mean flux path through the air gaps 23 and 24 is parallel to the pivot axis 2 of the fiexure hinge 31 for the armature lever 27.

FIG. 4 shows that the coils 17, 18, 19 and 20 are so connected in the electric circuit and to each other as to cause the flux on one side of the pivot 31 to so coact with the flux on the other side of the pivot as to produce a torque on the lever 27, Specifically, referring to FIG. 4, the coils 17 and 18, which are below the armature 21, are wired in a reverse direction with respect to the positive and negative sides of the supply current than the coils 19 and 20.

With the circuit shown in FIG. 5 of the prior Hilgert application, Ser. No. 389,435, now Patent No. 3,278,873 the carbon contacts 41, 43 have a hyperbolic resistance/ pressure characteristic curve-that is, the voltage gain across the contacts varies greatly from low to high pressure. This naturally tends to cause instability in the transducer at the low pressure-high gain portion of the curve, and sluggishness at the opposite end of the curve. In addition, there is a great resistance change at the time when the drop across the contacts is greatest, thus magnifying the instability at low pressures.

In order to improve the stability, the arrangements shown in FIGS. 5 and 6 of the present application have been conceived, each of which arrangements is capable of maintaining a constant low voltage drop across the contacts.

The detailed interconnections of the coils in the circuit and with one another are difiicult to show in diagram such as FIGS. 5 and 6, so FIG. 4 has been employed to illustrate the specific wiring of the coils with one another and with reference to the positive and negative sides of the current supply.

Referring first to FIG. 5, a supply voltage of approximately 4 volts DC is applied across a series circuit which includes the lines 46, 49 and 45, with the resistor 47 and semi-conductor diode 48 in said series circuit and the resistor being on the negative side of the diode. In this series circuit a voltage drop of approximately 0.3 volt is developed across the diode 48 and this is nearly independent of supply voltage.

Another series circuit includes the line 46, a line 51 and a line 52 leading to one of the transducer contacts that is on the positive side of the contact means 41, 43. This other series circuit also includes another line 53 leading from the other transducer contact to the emitter 54 of a transistor 55, includes a line 56 leading from the collector of the transistor to a junction point 57 in a line 58, includes through lines 58 and 59, includes the feedback coils 17-20 of the transducer, and includes the line 60 leading to the negative terminal of the supply voltage, the load 61 being in said latter line.

A condenser 62 is in a line 63 connecting junction points 50 and 57. Another condenser 64 is in a line 65 which connects with line 51 at junction point 66 and with line 58 at junction point 67. Part of the load current is bypassed through resistors 68 and 69 for calibrating purposes.

Junction point 50 is connected by a line 70 with the base 71 of the transistor. Thus the 0.3 volt drop across the diode 48 will result in the base 71 of the transistor being maintained at 0.3 volt more negative than the positive supply terminal.

Assuming the pressure on the contacts 41, 43 of the transducer to be at some intermediate value so that the contacts will be at an intermediate resistance, the base 71 of the transistor will be negative with respect to the emitter 54. Thus the transistor 55 will conduct, causing a current to flow through the contacts 41, 43, through the feedback coils 17-20 of the transducer, and through the load 61. There will be a voltage drop of approximately 0.2-0.3 volt across the contacts 41, 43.

If, however, increased pressure is exerted through the transfer lever 27 on the contacts 41, 43, the contact resistance will drop and the emitter 54 of the transistor 55 will become slightly more positive with respect to its base 71 so that the transistor will conduct more heavily. This action will increase the current through the contacts 41, 43 which will bring the contact voltage drop back to a value only slightly lower than its initial value. Thus the voltage drop across the contacts will be about the same as the voltage drop across the diode 48 and any voltage change which takes place must occur across the torque motor 17-28 and the transistor 55. The heavier current flow through the transistor 55 also flows through the torque motor of FIG. 2, there being an action in the nature of a negative feedback from the magnetic flux, as shown in FIG. 4, and ultimately a force balance is established between the action of the pneumatic force applied through 34 and the action of the torque motor, all as is fully described in co-pending application of Hilgert, Ser. No. 389,435, now Patent No. 3,278,873.

If resistance of the contacts 41, 43 rises, then the transistor 55 conducts less current to return the voltage across the contacts 41, 43 to near the former value. As the pressure on the contacts approaches zero the voltage drop across the contacts is limited by the voltage drop across the diode. Hence the load current follows the input pressure just as in the device of the co-pending Hilgert application heretofore referred to. However, with the present invention this excess voltage is dropped across the transistor 55 rather than across the contacts 41, 43, which contacts of the present invention have only a 0.2-0.3 voltage drop across them.

The condensers 64 and 62 help to stabilize the circuit against mechanical shock, vibration, and line voltage fluctuations. Condenser 62 acts as a negative feedback to make the base 71 of the transistor more positive if there is a sudden increase of current caused by vibration or shock. The condenser 64 primarily subdues system oscillations by adding a slight delay in the system response.

The form of circuit of FIG. 6 is also arranged so as to maintain a consistent low voltage drop across the contacts. However, in this circuit the output current does not pass through the contacts. This circuit is therefore particularly adapted for use Where thereis a need for a higher current output than the contacts would endure.

A supply voltage from the positive terminal 72 and negative terminal 73 of a suitable supply source is applied across three series circuits which are in parallel with one another; The first of these circuits consists of the line 74 with the semiconductor diode 75 and resistor 76. In this particular example the voltage is 18 volts, the resistor is 1500 ohms, and the diode is 1N462A. There is a voltage drop of approximately 0.7 volt across the diode 75, which drop is nearly independent of supply voltage.

The supply voltage is also applied to the series circuit which consists of the transducer contacts 41, 43, a transistor 77, and a resistor 78. In this example the resistor 78 is 10,000 ohms and the transistor is 2N404. The 0.7 voltage drop across the diode 75 is applied to the base 79 of the transistor through a line 80 which connects with the line 74. This arrangement maintains the base 79 of the transistor 0.7 volt more negative than the positive supply terminal 72.

The supply voltage is also applied across a third series circuit which consists of the feedback coils 17-20 of the transducer shown in FIGS. 1-4, a second transistor 81, and a resistor 82 which may be the load. In this example the resistor is 1,000 ohms and the transistor 81 is 2N1304. The line 83 forming the intermediate series circuit is connected by a line 84 with the base 85 of the transistor 81, there being a resistor 86 in the line 84. In addition, there is a line 87 in parallel with the resistors 78 and 82 with a capacitor 88 in said line.

Any voltage drop developed across the resistor 78 will appear between the negative supply terminal 73 and the base 85 of the transistor 81. This causes a voltage rise nearly equal to the drop across the resistor 78 to appear across the resistor 82 because of the emitter-follower action of the transistor 81. Any change in voltage across the resistor 82 will cause a proportional change in the current through the feedback coils 17-20 of the torque motor.

Assuming the contacts 41, 43 to be at some intermediate resistance, the base 79 of the transistor 77 will conduct, causing a voltage drop of approximately 0.7 volt across the contacts 41, 43. The current through the contacts (0.7 R contact). also flows through the resistor 78 developing a voltage drop (10,000 0.7/R) across 78 which is proportional to the current through the contacts 41, 43. This voltage is applied through the resistor 78 (10,000 ohms) to the base of the transistor 81 and simultaneously to the smoothing capacitor 88 mfd.). The base 85 of the transistor 81 is now positive with respect to its emitter and will conduct. Inasmuch as the transistor 81 is in an emitter-follower circuit there will be developed a voltage across 82 which is nearly equal to the voltage across the capacitor 88. Also the .current through the resistor 82 flows through the feedback coils 17-20 of the torque motor of FIGS. 1-4.

To follow a typical operation of the circuit, it may be assumed that there is some intermediate contacts resistance in the contacts 41, 43 and that both transistors '77 and 81 are conducting. If an increase of pressure causes the contact resistance at 41, 43 to drop, then the emitter of the transistor 77 becomes slightly more positive with respect to its base 79 and will conduct heavier. This will increase the current through the contacts 41, 43 which will bring the contact voltage drop back to a value only slightly lower than its initial value. This transistor current which flows through the resistor 78 causes a proportional change in voltage drop across the resistor and therefore a proportional drop across the capacitor 88. Any change in voltage across the capacitor 88 produces a proportional change in the voltage across the resistor 82 and also a like change in the current through the feed-back coils 17-20 of the torque motor. Thus the current of the torque motor of FIGS. 1-4 follows the contact pressure, and the torque motor flux provides feedback pressure to buck a portion of the contact pressure.

The transducer output can either be the current through the load 82 or the voltage drop across it, and may be as great in magnitude as the dissipation ratings of the transistor 81 and torque motor permit. Hence the dissipation limitations of the contacts 41, 43 are of no importance.

From the above it is apparent that in both the circuits of FIGS. 5 and 6 a consistent low voltage drop across the transducer contacts is maintained to greatly improve the stability of the transducer, which limitation of voltage drop also serves to minimize wear on the contacts. It is also clear that with the form of the invention of FIG. 5, which is suitable for small current output requirements, the contacts carry the full load current as they are in series therewith. Thus no dust or film build-up can occur as any deposit is quickly dissipated at each contact operation.

In the form of the invention of FIG. 6 the output current does not pass through the contacts. This arrangement is particularly suitable where there are higher current output requirements than the contacts 41, 43 could endure.

Various changes and modifications may be made without departing from the spirit of the invention, and all of such changes are contemplated as may come within the scope of the claims.

What I claim is:

1. In combination with a pressure-electric transducer having an electric circuit and having input signal mechanism which includes pressure-responsive electric contact means, the resistance of which decreases in accordance with pressure thereon to thereby elfect the voltage drop across said contact means, said transducer including magnetic feedback means having a coil in said circuit, control means including said pressure-responsive contact means controlling the flow of current through said feedback coil, the improvement comprising:

voltage limiting means connected in parallel with said contact means and feedback coil and including a diode;

transistor means having a first terminal connected to the negative side of said contact means and having a base, there being a connection between said base and the negative side of said diode;

the positive side of said diode being connected to the positive terminal of said contact means to cause a drop in voltage across said contact means whereby a constant low voltage drop across the contact means is maintained to stabilize the transducer performance; and

means between the feedback coil of the transducer and another terminal of said transistor for activating said feedback coil.

2. Apparatus as claimed in claim 1 in which said diode is a semi-conductor diode.

3. Apparatus as claimed in claim 1 in which said means which is between the feedback coil of the transducer and said other terminal of the transistor includes a second transistor having its emitter and collector in series with the feedback coil and having its base connected to said other terminal of the first transistor.

4. Apparatus as claimed in claim 3 in which said second transistor is connected in such a way that its emitter is negative with respective to its collector.

5. Apparatus as claimed in claim 1 in which there are three parallel series circuits connected to the supply, the first of which includes the voltage limiting means, the sec ond of which includes the contact means and transistor, and the third of which includes the feedback coil of the transducer and the load.

6. Apparatus as claimed in claim 5 in which the voltage limiting means in the first circuit includes a resistor in series with the diode; in which the second circuit includes the contact means, transistor and a resistor; and in which the third circuit includes a second transistor in series with the feedback coil and load, there being a joining connection between the base of said second transistor and the second circuit at a junction point between one side of the resistor and the transistor of said second circuit, and there being a resistor in said joining connection.

7. Apparatus as claimed in claim 6 in which there is condenser means connected between the base of said second transistor and the negative side of said load to lessen system oscillations by delaying the feedback signal.

'8. In combination with a pressure-electric transducer having an electric circuit and having input signal mechanism which includes pressure-responsive electric contact means, the resistance of which decreases in accordance with pressure thereon to thereby efiect the voltage drop across said contact means, said contact means having positive and negative terminals, and said transducer including magnetic feedback means having a coil in said circuit in series with said contact means, the improvement comprising:

voltage limiting means connected in parallel with the contact means and including a diode;

a transistor in said circuit between the negative terminal of said contact means and said magnetic feedback coil and in series therewith, said transistor having a base and there being a connection between said base and the negative side of said diode;

the positive side of said diode being connected to the positive terminal of said contact means to cause a drop in voltage across said contact means whereby a constant low voltage drop across the contact means is maintained to stabilize the transducer performance.

9. Apparatus as claimed in claim 8 in which the diode is a semi-conductor diode.

10. Apparatus as claimed in claim 8 in which said voltage limiting means includes a resistor in series with the diode.

11. Apparatus as claimed in claim 10 in which the base of the transistor is connected between the resistor and diode of the voltage limiting means.

12. Apparatus as claimed in claims 8 in which there is a load in series with the feedback coil, transistor, and contact means located between said coil and the negative side of the supply.

13. Apparatus as claimed in claim 8 in which there is condenser means connected with the transistor base for driving the transistor base toward a positive condition if there is a sudden abnormal increase in current.

14. Apparatus as claimed in claim 13 in which there is a second condenser means connected with the collector of the transistor and with the first condenser means to prevent a sudden drop in the coil current if the transistor current suddenly drops.

References Cited UNITED STATES PATENTS 2,383,757 8/1945 Ziebolz 340-l86 X 1/1950 Jones 340-177 X 

1. IN COMBINATION WITH A PRESSURE-ELECTRIC TRANSDUCER HAVING AN ELECTRIC CIRCUIT AND HAVING INPUT SIGNAL MECHANISM WHICH INCLUDES PRESSURE-RESPONSIVE ELECTRIC CONTACT MEANS, THE RESISTANCE OF WHICH DECREASES IN ACCORDANCE WITH PRESSURE THEREON TO THEREBY EFFECT THE VOLTAGE DROP ACROSS SAID CONTACT MEANS, SAID TRANSDUCER INCLUDING MAGNETIC FEEDBACK MEANS HAVING A COIL IN SAID CIRCUIT, CONTROL MEANS INCLUDING SAID PRESSURE-RESPONSIVE CONTACT MEANS CONTROLLING THE FLOW OF CURRENT THROUGH SAID FEEDBACK COIL, THE IMPROVEMENT COMPRISING: VOLTAGE LIMITING MEANS CONNECTED IN PARALLEL WITH SAID CONTACT MEANS AND FEEDBACK COIL AND INCLUDING A DIODE; TRANSISTOR MEANS HAVING A FIRST TERMINAL CONNECTED TO THE NEGATIVE SIDE OF SAID CONTACT MEANS AND HAVING A BASE, THERE BEING A CONNECTION BETWEEN SAID BASE AND NEGATIVE SIDE OF SAID DIODE; 